A braking band composite structure of a brake disc, particularly intended for a braking system of a brake disc of a vehicle, comprises an annular structure, or braking band, fixed to the rotating portion of a suspension of a vehicle, for example a wheel hub, and is provided with opposite braking surfaces suitable for cooperating with friction elements, or brake pads, seated in at least one calliper body arranged astride of such braking band and integral with a non-rotating component of the vehicle suspension. The controlled interaction between the opposite brake pads and the opposed braking surfaces of the braking band by friction determine a braking action which allows the deceleration or stop of the vehicle.
One of the major problems exhibited by the structures intended for the braking bands is that of extending the life of the braking surfaces.
As known, in fact, in the braking surfaces, especially in the braking surfaces of brake discs mounted on high performance or racing vehicles, tracks or other surface irregularities quickly create, for example due to the dirt or to the same friction material of the pads pulverized during the braking, which interpose between the braking surface and the pad during the braking action. These surface irregularities cause an irritating noise or rattle and a considerable increase of the wear of the disc and of the pad itself. In the practice, these disadvantages limit the braking disc life both for an inadequate service comfort and for excessive component wear.
The need of having a braking band of a brake disc with braking surfaces that besides being planar and parallel are smooth, that is, with a low surface roughness value, is therefore especially felt.
This problem is also felt in braking band structures of composite material, which are increasingly used on racing cars, but also on medium-high performance v vehicles.
In particular, this problem is felt in composite ceramic structures of braking bands of brake discs.
The term “composite ceramic structure” means all the structures obtained by:                carbonizing any mixture comprising resins, for example phenolic resins, graphite, for example in powder, and filaments or bundles of filaments of carbon fibers and        densifying the resulting porous structure with silicon infiltrations, thus obtaining a structure or matrix comprising Carbon (C), Silicon (Si) and Silicon Carbide (SiC).        
The opposite braking surfaces, suitably processed, of the braking band structure of these composite ceramic materials exhibit Silicon (Si), Silicon Carbide (SiC) and, buried or partly buried therein, bundles of carbon fibers (C). The braking surfaces of these Si and SiC structures therefore also exhibit Carbon fibers C exposed to oxidation.
In particular, these fibers are quickly oxidized when, during the braking action, the temperature of the friction surfaces increases, especially if an intense braking action, like that produced in a high performance vehicle, is produced.
As the carbon fibers exposed to oxidation burn, holes or tracks more or less quickly form onto the braking surface, which determine a quick reduction of the driving comfort, as well as a sudden increase of the pad wear.
Recently it has been tried to solve this problem by adding, at the braking surfaces, a surface layer which includes micro-fibers in place of the carbon filament bundles, in an attempt at forming, due to the burning thereof, holes or tracks of limited size and such as to reduce the rattle during the braking action.
An example of disc for brake disc of this type is described in U.S. Pat. No. 6,723,193 (Martin, Roland).
Despite being advantageous from several points of view, this solution allows reducing the above problem but it does not allow eliminating it.
In fact, despite the good mechanical features, the composite ceramic material described above in any case exhibits, as serious disadvantage, an unacceptable loss of free carbon from the braking surfaces which implies the forming of surface cavities. This disadvantage is caused by the tendency of the material, especially at high operating temperatures, to undergo such surface oxidation as to cause the surface loss of carbon based material.
Moreover, in the pas a solution has also been suggested wherein a monolithic SiC layer, for example of thickness variable between 0.2 mm and 5 mm is added to the Carbon (C) matrix, Si and SiC.
An example of composite structure of this type is described in U.S. Pat. No. 6,818,085 (Behr, Thomas et al.), and, only as regards to structures with Carbon-Carbon (C—C) body, in U.S. Pat. No. 6,077,607 (Zornik, Miklavz).
This solution, despite being advantageous from several points of view, is satisfactory until the material comprising carbon filaments located below the monolithic SiC layer burns due to the passage or infiltration of oxygen, infiltration which in any case (as experimentally proved) also occurs through the monolithic layer. Due to these burns that are substantially located in an area underneath the monolithic layer and substantially parallel to the braking surface, the monolithic layer starts to separate (raise or flake off), thus breaking (cracking), to then separate from the underlying matrix Dumping away) forming also large lowered areas that cause a drastic limitation of the disc life.
Moreover, in the industrial practice, these layers cannot be made thinner than 0.6 mm -1 mm. With these very high thicknesses, considerable surface tensions occur in the interface between the surface layer and the matrix containing carbon which favor an even more frequent forming of surface cracks.
GB 1,311,537 (Bendix Corporation) discloses the application of a coating on a C—C matrix. Also in this case, despite being advantageous from several points of view, the proposed solution implies a limitation to the operating temperature of the disc. For this known solution, the limit operating temperature is given by the temperature that leads to the burning of the C—C matrix (approximately close to 550° C.) in any case caused by the diffusion (even if limited) of oxygen into the structure, above which the coating layer separates or detaches from the underlying matrix (in the practice the surface layer flakes off). The main cause is related to the fact that even though these oxide layers described in GB 1,311,537 are dense, they allow the oxygen diffusion and thus the oxidation of the underlying material arranged in the separation surface between the coating and the matrix (it is common to find large oxidized areas in the interface plane between the coating and the matrix which with time lead to the coating separation, since the C—C subject to oxidation is present below the entire surface layer).
It is also known to coat a disc with an aluminum based metal matrix with a protective layer of few hundreds microns (for example alumina).
An example of this type of embodiment is described in WO 92/05292 (Murphy, Martin).
This solution allows a safe operation of the brake disc structure up to the melting temperature of the aluminum or of the aluminum based metal alloy matrix. As this temperature is exceeded, the coating layer detaches or flakes off as it loses a steady anchoring base to the matrix.